Antidepressant drugs are reported to decrease in the resting metabolic rate and alter body mass of depressed patients. Increased body mass may be partly related to the decrease in metabolic rate. The change in energy budgeting that follows chronic drug treatment may be related to the behavior activating effects of antidepressant drugs. Chronic inhibition of type A monoamine oxidase (MAO) with the antidepressant drug clorgyline alters body mass in Syrian hamsters. This change in body mass was primarily due to an decrease in body lipid content. In the current studies we observed that the clorgyline decreases oxygen consumption the level of body temperature and food consumption in Syrian hamsters. Our working hypothesis is that the clorgyline-induced decrease in oxygen consumption and temperature are related to the diminished food intake that accompanies chronic drug administration. Our data are consistent with the interpretation that MAO inhibition decreases the temperature set-point of the hypothalamus.